I. Field
The present disclosure relates generally to electronics circuits, and more specifically to oscillator and phase-locked loop (PLL).
II. Background
Oscillators and phase-locked loops are integral parts of many electronics circuits and are particularly important in communication circuits. For example, digital systems use clock signals to trigger synchronous circuits (e.g., flip-flops). Transmitter and receiver systems use local oscillator (LO) signals for frequency upconversion and downconversion, respectively. Wireless devices (e.g., cellular phones) in wireless communication systems typically use clock signals for digital circuitry and LO signals for transmitter and receiver systems. The clock and LO signals are generated with oscillators and their frequencies are often controlled with phase-locked loops.
A voltage-controlled oscillator (VCO) is an oscillator having an oscillation frequency that may be varied with a control voltage. A VCO may be designed with the capability to vary its oscillation frequency over a wide range. This wide tuning range may be required to (1) ensure that the VCO can operate at a required frequency or a required range of frequencies and (2) compensate for changes in the oscillation frequency due to component tolerances, integrated circuit (IC) process variations, temperature, voltage supply variations, and so on.
A large VCO gain is typically needed to achieve a wide tuning range. The VCO gain is roughly equal to the tuning range of the VCO divided by the control voltage range for the VCO. A large VCO gain may cause various adverse effects. First, if the VCO is used in a PLL, then the large VCO gain typically results in a wide loop filter bandwidth, which may compromise loop stability. In order to keep the loop filter bandwidth fixed and avoid instability, either a small charge pump current and/or a large loop filter capacitor may be used in the PLL. This results in a reduced signal-to-noise ratio for the charge pump and an area penalty for the large on-chip capacitor. Second, the large VCO gain results in the noise from circuit blocks (e.g., charge pump and loop filter) within the PLL being amplified by a large gain, which results in more jitter at the VCO output. All of these adverse effects are undesirable. The issues related to large VCO gain are more pronounced in low voltage applications since the control voltage range is more limited.
There is therefore a need in the art for techniques to effectively handle large VCO gain and achieve good performance.